As will be appreciated herein below, except as otherwise indicated, alloy designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2006.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
In the art various crystallisation methods and apparatuses are used to refine a metal (here used as an abbreviation for metal alloy), which comprises an amount of a foreign element that is unacceptable for the final purposes of the refined metal. Such a foreign element may be present because in the metal made from metal ore, the primary metal, too much of the foreign element is present, or because recycled scrap metals comprises a too high concentration of a foreign element. For example aluminium scrap may contain foreign elements like Fe, Si or Mg at commercially unacceptable levels without being bound to mix it with primary metal comprising little of such an element. In fractional crystallisation metal crystals formed during partial solidification of the mother liquid used as a starting material, have a composition that differs from the composition of the mother liquid itself. The mother liquid may have a hypo-eutectic (see e.g. U.S. Pat. No. 4,273,627) or hyper-eutectic composition.
In the method known from U.S. Pat. No. 4,273,627 a hypo-eutectic molten metal comprising one or more foreign elements is cooled to achieve partial solidification. The molten metal is cooled to just above a eutectic temperature. The crystals that form in the molten metal have a purer composition than that of the molten metal that is used as a starting point. These crystals can then be separated from the remaining molten metal by means of a solid-liquid separation technique. This process however has the drawback that when the initial concentration of foreign elements is high the amount of purified metal obtained is relatively low and the amount of by-product generated is high. This means the fractional crystallisation method may not be economically feasible for e.g. purifying scrap.
Another purification method is by means of separation of foreign elements in which a hyper-eutectic molten metal comprising one or more foreign elements is cooled to achieve partial solidification. The molten metal is cooled to a temperature just above the eutectic temperature. The foreign element(s) solidify to form crystals comprising at least one foreign element and/or pure crystals of a foreign element which can then be separated from the molten metal using a solid-liquid separation technique. A hypo-eutectic molten metal can be made hyper-eutectic by the addition of certain elements as disclosed in U.S. Pat. No. 5,741,348. This method has the disadvantage that the remaining liquid product obtained is not very pure and thus is of relatively low value.
Crystallisation may also be carried out at the eutectic temperature resulting in the simultaneous production of purified metal crystals and crystals comprising a higher concentration of one or more foreign elements compared to the composition of the mother liquid, see e.g. WO-2005/095658-A1. In the method known from WO-2005/095658-A1 the molten metal is cooled to a eutectic temperature in order to simultaneously form purified metal crystals and crystals comprising at least one foreign element. Compared to the starting composition of the molten metal the purified metal crystals are depleted of foreign elements, while the foreign element(s) are concentrated in the other crystals comprising at least one foreign element. Then at least some of the crystals comprising at least one foreign element are separated from the purified metal crystals by using a solid-solid separation technique. This known purification method is beneficial in view of economic feasibility for e.g. purifying scrap such as recycled aluminium, because a relatively pure metal as purified metal crystals can be obtained in a high yield. The solid-solid separation step is exemplified by inter alia a liquid-solid pre-separation step wherein both types of crystals are separated from the molten metal as a mixture and then this mixture is added to molten salt with a specific density between that of the specific densities of the purified metal crystals and the crystals comprising at least one foreign element so that some of the crystals sink into the salt whilst the remainder float on the salt.
Other methods for solid-solid separation described in WO-2005/095658-A1 use centrifugal forces, an electromagnetic field and gas bubbles based floatation.
Alternative pre-separation steps for separating both types of crystals from the total amount of molten metal are also described, such as filtration, centrifugation and a salt layer contacting step using stirring. In this process both types of crystals form simultaneously and settle together in the molten metal thereby forming a mixture of crystals. Although various techniques have been described for separating this mixture into its components, it has appeared that these techniques render the process amongst others complex and laborious to carry out.
A problem associated with almost every crystallisation process is the separation of the purified metal crystals from the impure mother liquid. The mother liquid remains between the crystals produced and in the crystals themselves. This problem may be overcome at least partially by using multiple crystallisation steps and/or counter current washing steps of the mother liquid from the crystals by remolten pure liquid.
A commercially employed method of fractional crystallisation for refining a metal is used in the so-called Yunnan crystalliser. This crystalliser is used for refining a tin alloy by removing Pb from Sn. The molten tin alloy is fed into an elongated container having an open top and an inclined bottom, in said container a screw is slowly rotated. The surface of the molten tin alloy is cooled by spraying water, resulting in the crystallisation of refined tin alloy. These crystals crystallise in the molten tin alloy and are transported to the shallow part of the container. Due to a temperature difference over the length of the container, in the shallow part the crystals are partially molten again, resulting in purer crystals. This mechanism repeats itself several times, and eventually very pure crystals are removed. Molten tin alloy containing Pb is removed at the deep end of the container. In this way, tin alloy containing approximately 10% Pb can be refined into tin alloy containing approximately 0.05% Pb. This method for refining a metal by using the Yunnan crystalliser however cannot be used for all types of metal. One problem is that most metals have a melting point that is far higher than the melting point of the tin alloy for which the Yunnan crystalliser has been build. Because of the higher temperatures, the heat radiation is much higher (the heat radiation increases with the fourth power of the temperature in K) and the heat losses are much higher as well. As a result of this it is much more difficult to control the temperature in the crystalliser. Another problem is that for many metals the temperature difference between the crystallisation temperature of the metal alloy and the crystallisation temperature of the pure metal is very small, in the order of a few K. The Yunnan crystalliser cannot be used for such small differences in crystallisation temperature. A further problem is that the use of mechanical parts such as said screw poses problems in some metals, because the metals normally used for the screw dissolve in these molten metals. An even larger problem is that the crystals formed in the molten metal tend to adhere to the walls of the crystalliser or the screw.
An improved version of the Yunnan process, specifically for the purification of aluminium, uses several stages of suspension crystallisation with multiple separation steps in a settling column arranged between subsequent crystallisation vessels. See e.g. international patent application WO-2005/095658 or WO-2004/005558.
U.S. Pat. No. 4,133,517 discloses a method and device for the continuous purification of an impure metal in a column consisting of a slurry of metal crystals in metal mother liquid. The column has a relatively hot zone at the lower end of the column and a relatively cool upper zone. In the cold zone the temperature is such that liquid and metal crystals coexist. The hot zone has a temperature sufficient to allow for remelting metal crystals. A continuous vertically inverted temperature gradient is applied between the zones, such that the cool zone at the top of the column is chilled resulting in the formation of an amount of metal crystals, usually as a coherent mass. Then incipient melting of said mass occurs in the liquid metal in the cool zone of the column thereby releasing the metal crystals from said mass. The crystals settle due to their higher density into the lower hot zone, wherein melting of the crystals occurs and in this way the hot zone becomes enriched with the pure liquid metal. In normal operation impure feed metal is supplied to the column, while purified liquid metal is withdrawn from the lower hot zone and impurity-enriched metal is withdrawn from the upper cold zone. A disadvantage of this process is that increasing the production rate of the column by either increasing the throughput or increasing the cross-section tends to disturb significantly the stability of the column. The hot remelt of crystals at the lower end of the column has a lower density than the mother liquid. As a result hot plumes of remelt rise in the mother liquid, thereby destroying the counter current flow through the crystal mass, which counter current flow is essential for a proper operation of said purification process.
It is also to be noted here that in general moving mechanical parts in crystallisation processes and high pressure pumping of liquid metal in order to establish a kind of separation are less suited for metal purification on an industrial scale.
Therefore there is an ongoing need in the art for improvements of metal purification processes and devices.